1. Field of the Invention
The present invention relates to an exhaust device for a motorcycle including a plurality of primary exhaust pipes, which are connected respectively to each of cylinders of a multi-cylinder engine body mounted on a body frame, and a collecting exhaust pipe which is commonly connected to the downstream end portion of the primary exhaust pipes.
2. Description of Related Art
As a collecting exhaust pipe, which is commonly connected to the downstream end portion of a plurality of primary exhaust pipes, the collecting exhaust pipe known in Japanese Utility Model Laid Open No. 62-731 is configured so as to join a main joint plate, which is configured such that two outer wall portions forming one-quarter, respectively, of the outer wall of the collecting exhaust pipe, are continuous and are integrally provided on both ends of a partition wall portion, with a pair of sub-joint plates constituting the rest of the outer wall of the collecting exhaust pipe.
Generally, it is desirable that the outer shape of a collecting exhaust pipe is formed so as to have a tubular shape apparently similar to a plurality of primary exhaust pipes, to extend continuously and respectively along each of the primary exhaust pipes, and to be formed with a tubular undulation in such a shape as to join on the downstream side in order to enhance its external appearance. However, in the collecting exhaust pipe disclosed in the above Japanese Utility Model, a partition wall portion has only to be disposed on the top portion of a swelling portion, which is swollen toward the inside of the collecting exhaust pipe out of the tubular undulation, and to be disposed in the center portion in the width direction of the collecting exhaust pipe, which thereby reduces the degree of freedom in disposition of the partition wall portions. Therefore, in the device set forth in Japanese Utility Model Laid Open No. 62-731 it is difficult to respond to the case where it is required to set the location in which the exhaust gas meets in the collecting exhaust pipe closer to the upstream side according to the characteristics of the vehicle. It is also possible to cause the increase in the number of parts due to the methods taken by connecting a collecting exhaust pipes and the like.
In addition, in the collecting exhaust pipe disclosed in the Japanese Utility Model mentioned above, the joining line of the main joint plate with two sub-joint plates undesirably occurs in the center portion in the width direction of a collecting exhaust pipe. Thus, it is required that a collecting exhaust pipe of the motorcycle has not only the function to cause the exhaust gas to meet inside an exhaust device so that the exhaust device constitutes a part of the vehicle external appearance, but also the functional beauty achieving such a smooth external design that the external flow passage shape from a plurality of primary exhaust pipes to secondary exhaust pipes (in smaller number than that of the primary exhaust pipes) or to an exhaust muffler is collected at parallel and equal spaces and then evenly collected. Accordingly, it is required to achieve the structure that can prevent the occurrence of the above-mentioned joint line.